1. Field of the Invention
The present invention relates to an air-conditioning system for an electric car and a method of controlling the air-conditioning system, and in particular to temperature control in heater operation.
2. Description of the Related Art
Generally, a car is equipped with an air-conditioning system to keep the interior comfortable and windows clear. The air-conditioning system has functions of cooling, heating, dehumidifying, demisting and defrosting.
A known air-conditioning system of a car powered by an internal combustion engine uses a refrigeration cycle equipment for cooling and dehumidifying, and a heater or waste heat from the engine for heating. Thus a car powered by an internal combustion engine can use waste heat from the engine to heat the car. Jpn. Pat. Appln. KOKAI Publication No. 2000-185548 discloses an air-conditioning system for a car powered by an internal combustion engine. In the air-conditioning system, the capacity of the refrigeration cycle equipment is decreased to prevent a decrease in the temperature of airflow from outlet ports in heater operation when the waste heat from the engine is low when idling, for example.
In contrast to a car powered by an internal combustion engine, an electric car is powered by a traction motor using electrical energy supplied by a battery. The heat produced by the traction motor and battery is less than that produced by an internal combustion engine. Thus, an electric car cannot use waste heat, i.e., the heat of the traction motor to heat the car.
Therefore, an air-conditioning system of an electric car uses electrical energy stored in a battery. For example, an air-conditioning system of an electric car uses a refrigeration cycle equipment using a compressor powered by a motor, for cooling and dehumidifying. An air-conditioning system of an electric car conditions the interior of the car by heating air or water as catalyst by an electric heater, and discharging the temperature-controlled air by means of a blower.
The above air-conditioning system of an electric car has a temperature control dial in the interior of the car. An air-conditioning system of an electric car is configured to set an interior temperature by the temperature control dial. A temperature control dial is also provided in an air-conditioning system of a car powered by an internal combustion engine.
The above air-conditioning system of an electric car has the following problems. Electrical energy stored in the battery is used to power the air-conditioning system. The electrical energy is also used to power the traction motor. As the electrical energy is used for the air-conditioning system, the range of the car is decreased.
Further, heat in the interior of the car is lost by motion-induced airflow while the car is traveling. Therefore, the temperature of a heat exchange medium for heater operation of an air-conditioning system of the car is set to a temperature, so that the interior of the car can be conditioned to a target temperature even while the car is traveling. In other words, the interior of the car is set to a temperature, assuming that the heated air in the interior of the car escapes to the outside. A temperature in the interior of the car is set higher than a target temperature, assuming the escape of heated air to the outside.
In an air-conditioning system of a car powered by an internal combustion, the rotational speed of the internal combustion engine is decreased when idling. Heater output is reduced during idling, even if the temperature setting of the heater stays the same. Therefore, the air-conditioning system of a car powered by an internal combustion engine performs appropriate air-conditioning according to whether the engine is doing work or idling, assuming a decrease in passenger compartment temperature caused by the reduction of heater output during idling.
However, as an electric car is powered by the electrical energy of a battery, the air-conditioning system of the electric car can perform heater operation at a high temperature setting, assuming the escape of heat inside the car by motion-induced airflow even at idling. In other words, the air-conditioning system of an electric car always performs constant heater operation in either during running or at idling.
Therefore, in an electric car equipped with an air-conditioning system, the passenger compartment temperature at idling is higher than that when the motor is doing work. As heater operation is performed at a temperature setting for running even at idling, a passenger compartment temperature at idling is higher than a target temperature. Thus, unnecessary air-conditioning is performed, and the battery energy is wasted. In particular, an air-conditioning system of an electric car uses battery energy, and unnecessary air-conditioning decreases a running distance, and wastes electric energy.